Intravenous lysergic acid diethylamide (LSD) given to rabbits in doses from 1 to 100 mug per kilogram of body weight produced a dose-related increase in intraocular pressure and outflow facility. Minor changes in systemic blood pressure were observed, but respiration rate was accelerated, and mydriasis became pronounced at higher doses. Diacetyl morphine (heroin) was given intravenously in doses from 0.1 to 2 mg. per kilogram of body weight. A dose-related decrease in intraocular pressure and an increase in outflow facility was found. A dose-related miosis was observed and at higher doses respiration became markedly depressed. Neither drug alters the permeability of the isolated ciliary epithelium. Both drugs appear to increase capillary blood pressure and, hence, aqueous humor inflow to cause the intraocular pressure to be maintained at approximately normal levels in face of increases in outflow facility of 50 per cent.